T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive leukemia of immature T-cell progenitors that has a peak incidence in older children and young adults. Although the use of intensified chemotherapy regimens has improved outcomes, T-ALL remains fatal in 20% of children and 50-70% of adults. Our recent work, together with that of Dario Campana's group at St. Jude, has demonstrated that differentiation arrest at the earliest stages of T-cell development predicts a very high risk of treatment failure n T-ALL. These cases are associated with a gene expression signature implicating upregulation of the PI3K-AKT pathway, whereas PTEN deletions also predict treatment failure. Thus, T-ALL treatment failure is associated with AKT pathway activation and with MYC overexpression, a feature common to most cases of T-ALL. We have now shown that MYC and AKT pathway activation both repress mitochondrial apoptosis in a conditional zebrafish model of T-ALL Thus, our central hypothesis is that an apoptotic blockade downstream of MYC and AKT mediates treatment resistance in high-risk T-ALL. However, the mechanisms through which MYC and AKT activation repress mitochondrial apoptosis remain undefined. In Aim 1, we will exploit the zebrafish model system to test the degree to which repression of BIM underlies apoptosis resistance in high-risk T-ALL with MYC and AKT activation. In Aim 2, we will combine biochemical analyses with therapeutic studies in high-risk human T-ALLs grown in immunodeficient mice to identify and overcome mechanisms of apoptosis resistance in high-risk human T-ALL. The successful completion of the work proposed here is expected to lead to the identification of mechanisms of resistance to chemotherapy-induced apoptosis in T-ALL cells, as well as to development of therapeutic strategies to overcome these. Given that most patients with high-risk T-ALL cannot be cured with current treatment regimens, the development of novel effective therapies for high-risk T-ALL is expected to represent a major therapeutic advance for these patients.